The seven wonders of TRAPPIST-1

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ULiège astronomers discovered seven telluric planets around the star TRAPPIST-1. “The TRAPPIST-1 system is the largest treasure of terrestrial planets ever detected around a single star.” This discovery, published this week in the journal Nature, revives the quest for life in the Universe.


iscovered in May 2016 by an international team led by par Michaël Gillon, an astronomer at the University of Liege, the exoplanetary system TRAPPIST-1 has revealed more secrets.
The intensive surveillance of the system using several telescopes, including NASA’s Spitzer space telescope, has not only reinforced the results received previously by the TRAPPIST-Sud telescope in Liege, but has also revealed the presence of four planets in the system bringing their number up to seven. According to the results published in the journal Nature (1), these seven planets are similar in size to Earth and could have liquid water on their surface, particularly in the case of three of them which orbit in the so-called “habitable” zone of their star. TRAPPIST-1 is the system with the biggest number of rocky planets and the greatest number of potentially habitable worlds ever discovered up to the present day. It has reignited the quest for extra-terrestrial life in the Universe.

This discovery was made in the context of the SPECULOOS project, a new more ambitious project for the detection of potentially inhabitable exoplanets. Directed by Michael Gillon, this project, supported by a Starting grant from the European Research Council (ERC), is in the final preparation phase on the site of the European Southern Observatory at Paranal (ESO) in Chile.

A fascinating exoplanetary system

The story began at the end of 2015 when Michael Gillon and the members of his team, including Emmanuël Jehin, an astronomer at ULiège, decoded the data acquired by the Liege telescope TRAPPIST-Sud, located in Chile. They had just observed a new exoplanetary system (2). Given the name TRAPPIST-1, and detected by means of the transiting method, the system already presented some very interesting characteristics, notably, the presence of three planets with a size and temperature similar to Earth and suitable for detailed study of their atmospheric conditions using the current technology available. The new observations made in 2016 by the team only served to increase this interest by gradually revealing other planets. Their exact number and their orbital characteristics remained unclear until autumn 2016, when the team used NASA’s Spitzer space telescope to continuously observe the system during a three-week period. These space observations revealed that there were in fact seven planets orbiting the star. These planets, called TRAPPIST-1b, c, d, e, f, g and h in order of increasing distance from their star, are all of a similar size to Earth. “The TRAPPIST-1 system is the greatest treasure trove of Earth-sized planets ever detected around a star”, says Michaël Gillon. “It is a fascinating planetary system, not only for the number of planets it has, but also because these planets present characteristics which are very similar to those on Earth”!

According to the results published in the journal Nature, three of the planets are in the habitable zone of the system (3). By “habitable zone” we mean the range of distances from a star at which a rocky planet like Earth can harbor large quantities of liquid water on most of its surface. And the existence of water means the existence of life, at least this is the case on Earth! “By way of comparison”, continues Michael Gillon, “our solar system contains two Earth-sized planets only one of which is in the habitable zone. With its seven Earth-sized planets, three of which are in its habitable zone, TRAPPIST-1 seems to be an incredibly rich planetary system. Studying it will be even more fascinating”!

The TRAPPIST-1 star is an ultracool dwarf, that is to say, a much smaller and colder star then the Sun. This explains why, even though they are much closer to their star than the Earth is to the Sun, the planets of the TRAPPIST-1 system could contain liquid water on at least part of their surface. In this respect, the three planets e, f and g, which orbit in the habitable zone, are the most promising. “While the other planets of the system can only contain water on a small part of their surface, these three planets e, f, and g could have oceans similar to those on Earth”!, explains Julien de Wit, a graduate of ULiège who is currently doing a PhD at MIT.

Next step : determinate the atmospheres' composition

Thanks to their current data, the researchers have been able to very precisely measure the radius of the planets, but also to obtain a first estimate of the mass of six of them. “We are continuing to intensely observe the system from the ground and from space”, explains Emmanuël Jehin, an astronomer at the University of Liege and co-head of the TRAPPIST project, “And soon we should have sufficiently precise masses to be able to establish the composition of the planets. In addition, we hope to determine the existence and extent of their atmospheres very soon”. These observations will be possible by means of the Hubble space telescope, but we will have to wait for the launch of the James Webb Space Telescope, expected in 2018 by NASA and the ESA, to carry the research further in the hope of determining the composition of these atmospheres or even to detect chemical signs of biological activity on the surface of these planets. “We will be able to compare the planets to each other”, continues Michaël Gillon, “and conduct what is called comparative planetology. In the next five to ten years, we should know a lot more about these planets: what they are composed of, how they were formed, what their surface conditions are, etc. And who knows? Perhaps we will have detected signs of life on one or several of them? If so, we will never look at the stars in the same way again…”

This discovery marks an important turning point in space exploration and makes TRAPPIST-1 an important target in the search for extra-terrestrial life in the Universe, these worlds represent our best chance of detecting life elsewhere at the current time.

(1) Gillon, E. Jehin, et al., Temperate Earth-sized planets transiting a nearby ultracool dwarf star, Nature, May 2016.

(2) Un trio de Terres à 40 années-lumière


Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1', Nature, 23/02/2017


Michaël Gillon1, Amaury H. M. J. Triaud2, Brice-Olivier Demory3,4, Emmanuël Jehin1, Eric Agol5,6, Katherine M. Deck7, Susan M. Lederer8, Julien de Wit9, Artem Burdanov1, James G. Ingalls10, Emeline Bolmont11,12, Jeremy Leconte13, Sean N. Raymond13, Franck Selsis13, Martin Turbet14, Khalid Barkaoui15, Adam Burgasser16, Matthew R. Burleigh17, Sean J. Carey10, Chris M. Copperwheat18, Laetitia Delrez1,4, Catarina S. Fernandes1, Daniel L. Holdsworth19, Enrico J. Kotze20, Aleksander Chaushev17, Valérie Van Grootel1, Yaseen Almleaky21,22, Zouhair Benkhaldoun15, Pierre Magain1, Didier Queloz4

  1. Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Belgium
  2. Institute of Astronomy, Madingley Road, Cambridge, UK
  3. University of Bern, Center for Space and Habitability, Sidlerstrasse, Switzerland
  4. Cavendish Laboratory, Cambridge, UK
  5. Astronomy Department, University of Washington, Seattle, USA
  6. NASA Astrobiology Institute's Virtual Planetary Laboratory, Seattle, USA
  7. Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA
  8. NASA Johnson Space Center, Houston, Texas, USA
  9. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, USA
  10. Spitzer Science Center, California Institute of Technology, Pasadena, CA, USA
  11. NaXys, Department of Mathematics, University of Namur, Namur, Belgium
  12. Laboratoire AIM Paris-Saclay, CEA/DRF – CNRS – Univ. Paris Diderot – IRFU/SAp, France
  13. Laboratoire d'astrophysique de Bordeaux, Univ. Bordeaux, CNRS, France
  14. Laboratoire de Météorologie Dynamique, Sorbonne Universités, UPMC Univ Paris 06, CNRS, Paris, France
  15. Laboratoire LPHEA, Oukaimeden Observatory, Cadi Ayyad University/FSSM, Marrakesh, Morocco
  16. Center for Astrophysics and Space Science, University of California San Diego, CA, USA
  17. Leicester Institute for Space and Earth Observation, Dept. of Physics and Astronomy, University of Leicester, UK
  18. Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK
  19. Jeremiah Horrocks Institute, University of Central Lancashire, Preston, UK
  20. South African Astronomical Observatory, Cape Town, South Africa
  21. Space and Astronomy Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
  22. King Abdulah Centre for Crescent Observations and Astronomy, Saudia Arabia

about michaël gillon

about emmanuël jehin

About the TRAPPIST telescopes

Installed in 2010 on the site of the Very Large Telescope (VLT) of the Southern European Space Observatory (ESO) of Silla in the Chilean desert of Atacama, le TRAPPIST-Sud telescope has, since 2016, had a little brother, the TRAPPIST-Nord telescope installed on the Observatory of Oukaimeden in Morocco. Financed by the FNRS and the University of Liege, the two telescopes have a mission to detect exoplanets (head: Michaël Gillon), and the study of comets and other small bodies of the Solar System in the hope of gaining a better understanding of the genesis of our Solar System and our Earth in particular (head: Emmanuël Jehin).

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About the SPECULOOS project

The discovery of the TRAPPIST-1 system is part of a wider project called SPECULOOS (Search for habitable Planets EClipsing Ultra-cOOl Stars), which aims to detect more systems of this type. While the TRAPPIST telescopes are concentrating on around a hundred ultracool stars, the SPECULOOS telescopes will target around a thousand of them. The first SPECULOOS observatory, made up of four telescopes, is currently being installed on the European Southern Observatory of Paranal (ESO) in Chile. It will begin intensive research of planetary systems similar to those the researchers have just identified using the transiting method. “Our prototype project only targeted a small sample of ultracool stars. In this context, the discovery of TRAPPIST-1 suggests that Earth-sized planets are very frequent around stars of this size. SPECULOOS, which will observe ten times as much targets and with greater precision, should detect many more, placing itself at the frontline of research into the search for life elsewhere in the Universe”, says a delighted Michaël Gillon, at the head of this ambitious new project.

more information

Additional references


Michaël Gillon, Space sciences, Technologies and Astrophysics Research Institut, University of Liège


Emmanuël Jehin, Space sciences, Technologies and Astrophysics Research Institut, University of Liège


By the Press Service of the University of Liège


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